1. Technical Field
The present disclosure relates generally to a gas sensor element which may be installed in an exhaust pipe of an internal combustion engine to measure the concentration of a specified component of exhaust emissions of the engine, and more particularly to a gas sensor equipped with a solid electrolyte body which at least exhibits oxygen ion conductivity and has a pair of electrode layers formed on opposed surfaces thereof and an electrically activated heater. The disclosure also relates to a gas sensor equipped with the above type of gas sensor element and production method thereof.
2. Background Art
A gas sensor element is known which is disposed in an exhaust path extending from an internal combustion engine, such as automotive engine, to measure a specified gas component of exhaust emissions, such as oxygen, nitrogen oxide (NOx), ammonia, or hydrogen, for controlling the burning of fuel in the engine or an operation of an exhaust emission control system.
Japanese Patent First Publication No. H01-253649 discloses the above type of gas sensor element equipped with a solid electrolyte body and a heating element stacked on the solid electrolyte body. The solid electrolyte body has a measurement gas-exposed electrode and a reference gas-exposed electrode. The measurement gas-exposed electrode is formed on a surface of the solid electrolyte body to be exposed to gas to be measured (which will also be referred to as a measurement gas below). The reference gas-exposed electrode is also formed on another surface of the solid electrolyte body to be exposed to a reference gas chamber filled with a reference gas. The gas sensor element is produced by firing a sensor layer and a heater layer to make a laminate of the solid electrolyte body and the heating element. The heating element works to heat the whole of the solid electrolyte body quickly to activate it. This type of gas sensor element is usually called a planar gas sensor element.
Japanese Patent First Publication No. 2002-228626 discloses a solid electrolyte oxygen sensor element which is made of a laminate of a sensing portion, an insulating layer, and a heating portion stacked on the sensing portion through the insulating layer. The heating portion is equipped with a heating element working to activate the sensing portion to measure the concentration of oxygen correctly. The insulating layer avoids the leakage of current from the heating element to the sensing portion.
Japanese Patent Second Publication No. 06-048258 discloses an oxygen concentration sensor equipped with a hollow insulating ceramic cylinder, an oxygen concentration measuring device, a sheet assembly, and an insulating protective layer. The insulating ceramic cylinder has a closed end and an open end. The insulating ceramic cylinder also has defined therein a reference gas chamber leading to the open end. The insulating ceramic cylinder also has an opening formed in a peripheral wall thereof in communication with the reference gas chamber. The oxygen concentration measuring device is fit in the opening of the insulating ceramic cylinder and equipped with electrodes affixed to opposed surfaces thereof. One of the electrode faces outwardly of the insulating ceramic cylinder, while the other electrode faces inwardly of the insulating ceramic cylinder. The sheet assembly is made up of an insulating sheet and heater leads and electrode leads affixed to the insulating sheet. The heater leads and the electrode leads are formed by metallic films. The insulating sheet also has an opening. The sheet assembly is wrapped around the periphery of the insulating ceramic cylinder with the opening facing the oxygen concentration measuring device. The protective layer is porous and disposed over the closed end and the opening of the insulating ceramic cylinder. The insulating ceramic cylinder and the sheet assembly are fired together. One of the opposed surfaces of the oxygen concentration measuring device is exposed to the reference gas chamber, while the other surface is exposed to the gas to be measured through the protective layer.
FIGS. 8(a), 8(b), and 8(c) illustrates a gas sensor element 10g, like in Japanese Patent First Publication No. H01-253649. The gas sensor element 10g will also be described later as a comparative example No. 1.
The gas sensor element 10z is of a planar type and includes a heating element 140z, and a solid electrolyte layer 100, and a reference gas chamber 130z formed between the heating element 140z and the solid electrolyte layer 100. Air, which is highly electrically insulating, is admitted into the reference gas chamber 130z. The air in the reference gas chamber 130z obstructs transmission of heat, as produced by the heating element 140z, to the solid electrolyte layer 100z, thus resulting in a lag in activating the solid electrolyte layer 100z to measure the gas correctly.
The gas sensor element 10z is in the form of a planar plate which is typically susceptible to breakage due to thermal stress. It is, thus, necessary to increase the thickness of insulating layers 150z and 160z in order to improve the durability of the gas sensor element 10z. This, however, results in an increase in overall size of the gas sensor element 10z, which leads to a drop in thermal efficiency and an increased lag in activating the gas sensor element 10z. 
The oxygen gas sensor, as taught in the above described Japanese Patent First Publication No. 2002-228626, has the insulating layer between the sensing portion and the heating portion. The insulating layer is formed by firing a green sheet or using screen printing techniques.
Thinning the insulating layer in a production process thereof may cause defects such as pinholes to be developed. The measurement gas, thus, passes through the pinholes and reaches the heating element. The heating element may react with contaminants in the measurement gas and then sublimate, thus resulting in a deterioration thereof. Increasing the thickness of the insulating layer in order to increase the resistance of the heating element to the oxidation for ensuring a required lifespan thereof will result in an increase in overall size of the oxygen gas sensor. Lots of thermal energy is, thus, needed to heat the insulating layer. In other words, lots of time is consumed to heat and activate the solid electrolyte layer.
The oxygen concentration sensor, as taught in the above described Japanese Patent Second Publication No. 06-048258, is equipped with the insulating ceramic cylinder with the opening formed in the peripheral wall thereof. The opening has an inner shoulder serving as a seat on which the oxygen concentration measuring device made of a solid electrolyte body is fit. The oxygen concentration measuring device has the electrodes affixed to the opposed surfaces thereof. Such an arrangement of the oxygen concentration measuring device results in complexity of layout of the electrode leads, which may lead to cracks in the electrode leads, the insulating ceramic cylinder, and the solid electrolyte body when being fired.
Japanese Patent Second Publication No. 06-048258 also teaches the insulating sheet made up of two discrete sheets: one being a heater carrier sheet on which the heating element is formed, and the other being an electrode carrier sheet on which the heater leads and the electrode leads are formed. The heater carrier sheet and the electrode carrier sheet are affixed separately to the insulating ceramic cylinder, thus resulting in a lack in transmitting the thermal energy produced by the heating element to the electrode carrier sheet. This leads to a lag in activating the oxygen concentration sensor. Additionally, it is also necessary to affix the heater carrier sheet and the electrode carrier sheet to the insulating ceramic cylinder so as not to overlap each other. This contributes to inconvenience in production of the oxygen concentration sensor.